Rotation sensor using a radiation emitter and detector and a duty cycle feedback loop

ABSTRACT

A rotation sensor (5) comprises a fixed member (10) and a rotatable member (20). The fixed member (10) contains a radiation emitter (40) which emits radiation along a path (35). The rotatable member (20) rotates about an axis (65) substantially perpendicular to the path (35) and has a head containing slots (25) arranged to periodically transmit and block the radiation emitted by the emitter (40). The fixed member (210) also has a radiation detector (50) which detects radiation transmitted by the slots (25) of the rotatable member (20). Thus, the rotational movement of the rotatable member (20) with respect to the fixed member (10) is sensed by the frequency of the radiation detected by the radiation detector (50).

FIELD OF THE INVENTION

This invention relates to rotation sensors and particularly but notexclusively to optoelectronic rotation sensors.

BACKGROUND OF THE INVENTION

A conventional method for sensing vehicle speed involves a flexiblecable linking an axle or other moving element of the vehicle to ameasurement device such as a speedometer.

A known alternative to the above method is a device comprising a Halleffect sensor in combination with a moving magnet. The magnet may beconnected to the moving element of the vehicle, and the sensors to afixed element of the vehicle in close proximity to the magnet, such thatrotation of the magnet gives rise to an induced current in the sensors,which can be transmitted as electrical signals to the measurementdevice.

A problem with this arrangement is that the separation between north andsouth poles and magnetic field variations due to ambient temperaturegive rise to inaccurate and unstable transmitted signals.

Additionally, the mass of the magnet gives rise to a significant momentof inertia when rotating, which produces vibrations and leads toexcessive wear of the device.

Furthermore, it is necessary to maintain a constant distance between themagnet and the sensor. Any variation in distance leads to furtherinaccuracies in the signals.

This invention seeks to provide a rotation sensor which mitigates theabove mentioned disadvantages.

SUMMARY OF THE INVENTION

According to the present invention there is provided a rotation sensorcomprising a fixed member; a radiation emitter disposed on the fixedmember for emitting radiation along a path; a rotatable member arrangedto rotate about an axis substantially perpendicular to the path andcomprising a head having slots therein arranged in the path forperiodically transmitting and blocking, in use, the radiation emitted bythe emitter; and a radiation detector disposed on the fixed member fordetecting radiation transmitted by the rotatable member; whereinrotational movement of the rotatable member with respect to the fixedmember is sensed by the frequency of the radiation detected by theradiation detector.

The fixed member preferably further comprises an electronic circuitcoupled to send and receive electrical signals to and from the emitterand the sensor respectively. Preferably the electronic circuit includesa duty cycle feedback loop.

The duty cycle feedback loop is preferably selectively coupled to thesensor, such that for large rotations of the rotatable member the loopis coupled to the sensor and for small rotations of the rotatablemember, the loop is not coupled to the sensor.

Preferably the electronic circuit further comprises an output terminalfor providing signals indicating the speed of rotation of the rotatablemember. The radiation emitter is preferably a light emitting diode.

Preferably the fixed member further comprises a saddle enclosing theradiation emitter, the radiation sensor, the path and the head of therotatable member.

In this way problems of inaccurate and unstable transmitted signals andvibrations leading to excessive wear associated with Hall effect sensorsare reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be described withreference to the drawing in which:

FIG. 1 shows a preferred embodiment of a rotation sensor in accordancewith the invention.

FIGS. 2, 3 and 4 show in detail an end view of a rotatable memberforming part of the preferred embodiment of FIG. 1.

FIG. 5 shows a circuit diagram of an electrical circuit forming part ofthe preferred embodiment of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a rotation sensor 5 comprising ahousing 10 and a rotatable shaft 20. The housing 10 is a single piece ofmoulded material, such as strengthened plastic material, and furthercomprises a saddle 70.

A Gallium Arsenide Light Emitting Diode (LED) 40 is disposed within thesaddle 70 and has a connection 45 to a circuit 60 to be furtherdescribed below. The LED 40 is arranged to emit radiation along aradiation path 35.

A silicon radiation sensor 50, suitable for detecting the radiationemitted from the LED 40 is also disposed within the saddle 70, at aposition along the radiation path 35 substantially facing the LED 40.The radiation sensor 50 is thus arranged to receive and detect theradiation emitted along the radiation path 35. The radiation detector 50has a connection 55 to the circuit 60 to be further described below.

The rotatable shaft 20 is partially enclosed in the housing and arrangedto rotate about an axis 65, which is perpendicular to the radiation path35. A metal collar 30 is located between an aperture of the saddle 70and the rotatable shaft 20. In this way the metal collar 30 provides aseal for the saddle, reduces friction and maintains a substantiallyconstant relative position between the rotatable shaft 20 and the saddle70.

The circuit 60 is also located in the housing 10. A cover 75 protectsthe circuit 60 and seals the housing 10.

The rotatable shaft 20 further comprises a head containing a number ofslots 25. The rotatable shaft 20 is arranged such that the head islocated on the radiation path 35 between the LED 40 and the radiationdetector 50. A square axle 15 of the rotatable shaft 20 is located at anend remote from the head, for connection to an external drive shaft of avehicle (not shown). A threaded hole 85 on the housing 10 is used tosecure the rotation sensor 5 to a fixed body such as a drive shafthousing of a vehicle (not shown).

Referring also to FIG. 2, a transmission configuration 90 shows the headof the rotatable shaft 20 located in the radiation path 35 between theLED 40 and the radiation detector 50. The slots 25 are formed by radialapertures 110 and radial wedges 115 extending towards but not reachingthe centre of the rotatable shaft 20. The configuration 90 allowsmaximum radiation transmission from the LED 40 to the radiation detector50, without erroneous crosstalk transmitted via neighbouring slots.

Referring also to FIG. 3, a limit configuration 95 shows the head of therotatable shaft 20 rotated with respect to the transmissionconfiguration 90 to a position at the limit of transmission, where theradiation transmitted by the LED 40 is almost completely blocked by therotatable shaft 20, by virtue of the radial wedges 115.

Referring also to FIG. 4, a blocked configuration 100 shows the head ofthe rotatable shaft 20 further rotated with respect to the transmissionconfiguration, such that the radial wedges 115 of the rotatable shaft 20completely block the radiation path 35. The transmitted radiation iscompletely blocked, and cannot reach the radiation detector 50. Thepossibility of crosstalk between two non-aligned slots is eliminatedbecause the radial wedges 115 extend towards the centre of the rotatableshaft 20.

Referring also to FIG. 5, an electrical arrangement 150 shows thecircuit 60, the LED 40 and the radiation detector 50. The circuit 60includes a duty cycle feedback loop 210, comprising first and secondcomparators 175 and 185 respectively and first and second integrators180 and 190 respectively.

A current source 170 provides operating current for the LED 40. Anoutput signal 215 received from the radiation detector 50 via theconnection 55 is coupled to the first comparator 175 and compared to asignal 220 from the second integrator 190. The first comparator 175 isused to provide a logical squared output signal 165. The output signal165 is integrated with the first integrator 180 to provide a mean valuesignal 225. The mean value signal 225 is compared with a constantreference voltage 205 by the second comparator 185. An output signal 230of the second comparator 185 is integrated by the second integrator 190to provide the signal 220.

A frequency to voltage converter 195 is used to convert a furtherportion of the signal 165 frequency into the voltage 235. The voltage235 is compared to the reference voltage 205 by the comparator 200. Withthis arrangement when the signal 165 frequency is low, the voltage 235is low and when it is lower than the reference voltage 205, thecomparator 200 automatically switches off the duty cycle feedback loopand forces the threshold voltage 220 to a fixed value. The duty cyclefeedback loop is switch back on when the voltage 235 is greater than thereference voltage 205.

A power supply terminal 155, a ground terminal 160 and an output voltageterminal coupled to receive the output voltage 165 are all housed in aconnector 80, which provides connections to an external power supply andto a remote output unit (not shown) which converts the output signalinto a suitable speed display.

In operation, rotation of the external drive shaft coupled to the squareaxle 15 gives rise to rotation of the rotatable shaft 20 about the axis65. The slots 25 consequently produce alternate periods of interruptionand transmission of the radiation along the radiation path 35 to theradiation detector 50, giving rise to a frequency of interruption, whichis detected by the radiation detector 50 and indicates the speed ofrotation of the rotatable shaft 20.

With this arrangement the signal 165 duty cycle is proportional to theconstant reference voltage 205 regardless of the working conditions andenvironment (oil, temperature, pressure..) and component characteristicsdispersion. Any change in the output signal 215 automatically produces achange in the signal 220 to maintain the same duty cycle at the outputsignal 165.

As described above, the comparator 200 automatically switches off theduty cycle feedback loop and forces the threshold voltage 220 to a fixedvalue when the output signal 165 has a very low frequency. This preventsany oscillations when the rotatable shaft 20 is turning very slowly,leading to accurate speed measurement for very slow speeds.

The perpendicular arrangement reduces degradation of the accuracy of thedetection, because small rotations of the rotatable shaft 20 along theaxis 65 with respect to the saddle 70 make substantially no differenceto the interruption frequency.

It will be appreciated by a person skilled in the art that alternateembodiments to the one hereinbefore described are possible. For example,the housing could be fabricated from a material other plastic, and couldbe made from more than one piece.

Furthermore, the LED 40 could be of a type other than the GalliumArsenide LED described above, and the radiation detector 50 would bereplaced with a detector suitable for the type of LED chosen.

Moreover, the size and shape of the rotatable shaft 20 may be altered,and the metal collar 30 could be replaced by an alternative means formaintaining relative position between the rotatable shaft 20 and thesaddle 70.

I claim:
 1. A rotation sensor comprising:a fixed member; a radiationemitter disposed on the fixed member for emitting radiation along apath; a rotatable member arranged to rotate about an axis substantiallyperpendicular to the path and comprising a head having slots therein andarranged in the path for periodically transmitting and blocking, in use,the radiation emitted by the emitter; a radiation detector disposed onthe fixed member for detecting radiation transmitted by the rotatablemember; wherein rotational movement of the rotatable member with respectto the fixed member is sensed by the frequency of the radiation detectedby the radiation detector; and an electronic circuit coupled to send andreceive electrical signals to and from the emitter and the detectorrespectively, the electronic circuit including a duty cycle feedbackloop comprising:a first comparator having a first input coupled to anoutput of the radiation detector, a second input and an output, thefirst comparator for generating a logical squared output signal at itsoutput; and a second comparator having a first input coupled to theoutput of the first comparator, a second input coupled to a referencevoltage, and an output coupled to the second input of the firstcomparator, wherein, in use, the duty cycle feedback loop adjusts a dutycycle of the logical squared output signal so that the duty cycle has apredetermined value.
 2. The rotation sensor of claim 1 wherein the theelectronic circuit further comprises:a frequency to voltage converterhaving an input coupled to the output of the first comparator and anoutput; and a third comparator having a first input coupled to theoutput of frequency to voltage converter, a second input coupled to thereference voltage and an output for coupling to the second input of thefirst comparator, wherein the duty cycle feedback loop is selectivelycoupled to the radiation detector depending on the output of the thirdcomparator such that when the voltage signal at the output of thefrequency to voltage converter is greater than the reference voltage,the duty cycle feedback loop is coupled to the radiation detector andwhen the voltage signal at the output of the frequency to voltageconverter is less than the reference voltage, the duty cycle feedbackloop is not coupled to the radiation detector.
 3. The rotation sensor ofclaim 1 wherein the electronic circuit further comprises an outputterminal coupled to the output of the first comparator for providingsignals indicating the speed of rotation of the rotatable member.
 4. Therotation sensor of claim 1 wherein the radiation emitter is a lightemitting diode.
 5. The rotation sensor of claim 1 wherein the fixedmember further comprises a saddle enclosing the radiation emitter, theradiation detector, the path and the head of the rotatable member. 6.The rotation sensor of claim 1 wherein the slots are arrangedperpendicular to the axis and are formed by radial apertures and radialwedges extending perpendicularly towards the axis.